An acoustic wave device using a piezoelectric thin film resonator is used as a filter and a duplexer of a wireless device such as a mobile phone. The piezoelectric thin film resonator has a structure in which a lower electrode and an upper electrode sandwiches a piezoelectric film and face with each other.
As a wireless system rapidly spreads, many frequency bands are used. As a result, a demand for sharpening skirt characteristic of the filter or the duplexer is enhanced. Enlarging a Q value of a piezoelectric thin film resonator is one of methods for sharpening the skirt characteristic. Leakage of acoustic wave energy from a resonance region to outside is one of factors of degradation of the Q value of the piezoelectric thin film resonator.
Japanese Patent Application Publication No. 2006-109472 (hereinafter referred to as Document 1) discloses a technology in which the Q value is improved by providing a ring band on a surface of a lower electrode or an upper electrode.